The power plants which are connected to the supply network for feeding electric power are subjected to specific requirements for a trouble-free operation. This especially concerns the so-called primary control within the network, according to which power reserves have to be generated and fed to the network within a matter of seconds by means of the associated power plants for restoring normal operation in the event of a fault which is characterized, for example, by a drop in power and a drop in frequency which is associated therewith. Thus, on the part of the power plants it is necessary, for example, to additionally activate at least two percent of the respective power plant nominal output for primary control within a time span of 30 seconds at most.
In the case of steam power plants, it is customary to keep the inlet valves for respective steam-turbine stages in a throttled position for this purpose, in order to be able to utilize the steam reserve of the steam generator by means of spontaneous opening of the inlet valves during briefly occurring frequency deviations. In this way, it is possible to release stored power reserves and therefore counteract the frequency deviation within the network.
In addition to increasing power by means of increasing or decreasing the throttling of control valves, it is also known to shut down preheaters which are provided in the water-steam cycle of the steam turbine and are heated by means of bleed steam from the steam turbine. A stream of condensate which is directed through the low-pressure preheaters at the same time can be stopped within a few seconds and increased again. This measure for fast power control in fossil-fired power plant units by means of directed shutting down of the preheaters with condensate stopping is described, for example, in DE 33 04 292 C2.